Strange Storm Shaped by Gravity Wave Undular bores are a type of gravity wave, one in which gravity is the force that pulls the wave down. Coleman likens the cloud waves to those created when a boat moves across the water."When a boat goes tearing across a lake, water in front of the boat is pushed upward," he explained. "Gravity pulls the water back down again and this sets up a wave." The full story is here: [link to www.livescience.com]